The wavelength of light is related to its energy: the shorter the wavelength, the higher the frequency, and the higher the energy. X-ray radiation generally corresponds to wavelengths in the range 0.01 to 10 nanometers and energies in the range of 120 electron-volts (eV) to 120 keV.
X-rays are valued in medical imaging for their ability to penetrate soft tissue, and in materials science applications because they reveal physical (e.g., crystal) properties through diffraction and chemical composition through characteristic spectral (fluorescence) lines. X-ray wavelength or energy is typically adjusted to suit particular applications (e.g., a particular medical imaging technique on a particular type of tissue), but additional control capability is often desirable. For example, X-ray radiation used in medical imaging is classified as a carcinogen by both the World Health Organization and the U.S. government. Deleterious physical effects on tissue are proportional to the X-ray energy and exposure time. High-precision dose control through time-variation of the X-ray emitter is thus highly desirable to realize diagnostic or therapeutic outcomes while minimizing unintended side effects.
A calibrated X-ray detector is needed to measure the energies of X-rays; such spectroscopy is relevant to materials science and space (e.g., X-ray astronomy) applications. Most present methods of in-flight X-ray detector calibration use radioactive sources to generate X-rays of known energy. However, this calibration method greatly reduces the sensitivity of the X-ray detector because X-rays constantly produced by the radioactive source become a source of background “noise” that hinders the detection of cosmic X-rays. What is needed is a modulated X-ray source (MXS) that allows an X-ray detector to both remain properly calibrated and retain its sensitivity by producing X-rays only at pre-determined intervals and at a controllable rate.
Prior state-of-the-art MXS systems could generally vary their output intensity only on timescales of seconds in the ease of hot filament X-ray sources. There are also femtosecond X-ray pulsers, but they cannot produce output X-ray flux with arbitrary waveforms because the lasers that drive those sources have minimum pulse recovery and recycling times, and cannot arbitrarily and continuously vary the output intensity. Moreover, the output X-ray flux of most prior MXS technologies has been too small to be useful for many applications, while their size and input power needs have been too large to allow portability, again limiting their potential applications. Meanwhile, the cost to manufacture them is high.
Photoelectric X-ray sources have been proposed in the past for limited applications typically not involving high flux, rugged portability, or the need for arbitrary intensity variation. Previous photoelectrically driven X-ray sources typically use high-efficiency photocathodes, which are extremely unstable and cease to function after exposure to minimal quantities of oxygen. In addition, these high-efficiency photocathodes have considerable “dark current” (electron emission even in the absence of stimulating light), which would produce unwanted X-rays during “off” times.
Accordingly, it would be desirable to provide an X-ray source device that addresses at least some of the problems identified above.